Opinion | Dont Underestimate the Enduring Power of ISIS

On New Year’s Day, a confused, disgruntled and indebted veteran drove into a crowd of joyful celebrants in New Orleans, killing 14 and injuring 35 more.The assailant said shortly before the attack that he had joined the Islamic State, the brutal terrorist movement that at one point controlled an area in the Middle East the size of Britain.In its heyday, ISIS marketed itself as offering what one fighter called a “five-star jihad,” promising recruits a paradoxical mix of religious authenticity and material rewards, from free housing to a glamorous new identity to access to wives.

At its height, it was the wealthiest terrorist organization in modern history.Today, while the ISIS caliphate is gone, the group has cells and affiliates scattered across Africa, Southeast Asia, Afghanistan and Syria.It maintains an active online presence, and is still a threat: With the fall of the Assad regime in Syria, the authorities are concerned about a potential resurgence by ISIS there, while an offshoot in Afghanistan, ISIS-K, claimed responsibility for a significant attack last year in Russia and is believed to be behind another in Iran.But the twisted heart of the utopia ISIS was trying to build, and all that it claimed to offer, no longer exists.

So why would the group’s extreme ideology — rejected by the overwhelming majority of the world’s Muslims — appeal to a down-on-his-luck American veteran, five years after the caliphate’s fall?For 20 years, I’ve been studying Western recruits to domestic and transnational terrorist organizations.I’ve interviewed jihadis, white-nationalist terrorists and eco-terrorists to understand their motivations and to prevent future violence.

In my view, the appeal of some of the most crucial elements that ISIS offered to vulnerable or confused Western recruits — doctrinal certainty, identity, redemption and revenge — are as strong as ever, and will continue to resonate with people who can find it online.Most of us, as adult...